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In the Light of Evolution Volume IV: The Human Condition
remains attributed to Ar. ramidus share some features in common with living species of Pan, others that are shared with the African apes in general, and several dental and cranial features that it is claimed are shared only with later hominins, such as Au. afarensis. Thus, the discoverers have suggested that the taxon belongs within the human clade (White et al., 2009). The body mass of the presumed female partial skeleton has been estimated to be ca. 50 kg, the canines are claimed to be less projecting than those of common chimpanzees, and the degree of functional honing is modest. The postcanine teeth are relatively small, and the thin enamel covering on the teeth suggests that the diet of Ar. ramidus may have been closer to that of chimps/bonobos than to later hominins. Despite having ape-like hands and feet, the position of the foramen magnum and the reconstruction of the poorly preserved pelvic bone have been interpreted as confirmation that Ar. ramidus was an upright biped.
The type specimen of the taxon Orrorin tugenensis Senut et al. 2001 is BAR 1000’00, a fragmentary mandible, recovered in 2000 from the locality called Kapsomin at Baringo in the Tugen Hills, Kenya. The 13 specimens in the hypodigm all come from four ca. 6-Ma localities in the Lukeino Formation. The morphology of three femoral fragments has been interpreted as suggesting that O. tugenensis is an obligate biped (Senut et al., 2001; Richmond and Jungers, 2008), but other researchers interpret the radiographs and CT scans of the femoral neck as indicating a mix of bipedal and nonbipedal locomotion (Ohman et al., 2005). Otherwise, the discoverers admit that much of the critical dental morphology is “ape-like” (Senut et al., 2001).
Sahelanthropus tchadensis Brunet et al. 2002 is the taxon name given to fossils recovered in 2001 from the ca. 7-Ma Anthrocotheriid Unit at Toros-Menalla, Chad. The type specimen is TM266-01-060-1, a plastically deformed adult cranium, and the rest of the small hypodigm consists of mandibles and some teeth; there is no published postcranial evidence. S. tchadensis is a chimp/bonobo-sized animal displaying a novel combination of primitive and derived features. Much about the base and vault of the cranium is chimp/bonobo-like, but the relatively anterior placement of the foramen magnum is hominin-like. The supraorbital torus, lack of a muzzle, apically worn canines, low, rounded, molar cusps, relatively thick tooth enamel, and relatively thick mandibular corpus all suggest that S.tchadensis does not belong in the Pan clade (Brunet et al., 2002).
The most recently recognized taxon in the “possible hominin” grade category is Ardipithecus kadabba Haile-Selassie, Suwa, and White 2004 (Haile-Selassie, 2001; Haile-Selassie et al., 2004). The new species was established to accommodate cranial and postcranial remains announced in 2001 and six new dental specimens announced in 2004. All of the hypodigm were recovered from five ca. 5.8–5.2-Ma localities in the Middle